This invention relates to the transmissin lines having an inner conductor concentrically within an outer sheath with a gaseous insulation medium therebetween and particularly to arrangements in which solid insulators support the inner conductor in relation to the outer sheath and also support a particle trap in relation to the outer sheath.
General practice is for the inner conductor of a gas insulated transmission line to have support insulators at axially spaced locations. A typical support insulator is cast circumferentially around the inner conductor, or a metal sleeve into which the conductor is inserted, and has three uniformly spaced posts or legs, referred to as a tri-post arrangement, extending toward the outer sheath. One insulator, usually at an end of the conductor, is rigidly attached to the outer sheath. The outer ends of the posts of others of the insulators are preferably not rigidly attached to the outer sheath in order to permit relative axial movement between the inner conductor and outer sheath due to thermal effects. Also required is a particle trap in the area where there is contact between the insulator assembly and the outer sheath.
These various requirements have been met by prior art such as U.S. Pat. No. Re. 31,949, July 16, 1985, (original Pat. No. 4,335,267, June 15, 1982) by M. D. Hopkins, where an arrangement is shown and described in which wheel assemblies are located within support insulator openings in the lower two of the three insulator legs for facilitating axial movement. In a third leg is an insulating contact button cover allowing insertion of the conductor with low abrasion. When inserted, the button cover is removed for making electrical contact between a particle trap ring and the outer sheath. The structure requires cast-in metal inserts in the insulator legs which are undesirable as they complicate the structure and increase its expense. An earlier arrangement used polytetrafluoroethylene pads at the ends of the insulator legs. The pads were highly machined to provide a curved outer surface for sliding against the sheath and had an inner recess for fitting over a bolt in the insulator. This arrangement was not as successful as that of the mentioned patent. In general, the former structures such as that of the mentioned patent require several components which it would be desirable to reduce in number, cost and complexity. In addition, it is desirable to have an arrangement that may be readily assembled with relatively loose tolerances while ensuring reliable performance. Other aspects of gas transmission lines where simplicity and economy are achieved are described in copending application Ser. No. 938,910 filed Dec. 8, 1986 by Meyer et al. and assigned to the present assignee.
Briefly, in accordance with the present invention, a solid insulator is firmly attached to the inner conductor by being cast onto it (or a sleeve), as formerly, but the arrangement for associating the insulator to the outer sheath is simplified by requiring no cast-in metal inserts with their inherent risk of becoming loose or of causing insulator cracking due to overload. Instead, the insulator, which may be a tri-post or some other geometrical configuration, and an insulating bearing block are simply fit together. For example, the insulator may be provided with a simple blunt extremity that fits within an aperture of the insulating bearing block, preferably merely in a loose fit. The bearing block requires no special shaping so as to conform to the cylindrical sheath, although such shaping may be employed if desired. The block preferably is of a relatively low friction, abrasion resistant material such as high molecular weight polyethylene. Such a bearing block is suitably used on each leg or post of the insulator.
Additionally, the bearing block has a low dielectric constant compared to that of the support insulator. A particle trap ring is arranged so any particles produced during the assembly of the inner conductor and insulators within the outer sheath are confined in the trap. The particle trap ring and the sheath are connected electrically by a spring arm, extending out from the particle trap, that has a contact element on its end that runs against the outer sheath. The bearing block itself supports the particle trap and requires no additional components other than fasteners such as pop rivets for securing them together.
What is further advantageous is that the attachment of the particle trap to the bearing block can be at a location that is spaced from the support insulator because the bearing block may be extended axially sufficiently for that purpose. This means the bearing block eases the transition between the higher dielectric constant support insulator and the conductive particle trap and outer sheath so the arrangement is not prone to cause electrical breakdown even with loose tolerances.
It is therefore believed that the present invention has satisfactory or better performance characteristics in relation to previous structures while yielding considerable savings in parts cost and assembly time.